1. Technical Field
This disclosure generally relates to the field of transmission of audio and video streams in wireless local area network (WLAN) communications and, more particularly, to quality of service for multicast and broadcast of audio/video streams in WLAN communications.
2. Description of the Related Art
In recent years, wireless computer networks such as WLANs have been widely deployed in places ranging from homes and businesses to hotels, airports, schools, etc. Compared to wire networks, a WLAN provides users mobile connectivity and are typically more scalable and require lower cost in network construction. With the popularity of multimedia applications like streaming multimedia and voice over IP on the rise, it is important to ensure quality of service in the delivery of time-sensitive multimedia content in a WLAN.
Within the IEEE 802.11 WLAN standards, the IEEE 802.11e is an approved amendment that defines a set of quality of service enhancements for WLAN applications through modification to the media access control (MAC) layer. Specifically, the IEEE 802.11e defines two access protocols at the MAC layer: the mandatory distributed coordination function (DCF) and the optional point coordination function (PCF).
The DCF protocol uses a carrier sense multiple access with collision avoidance (CSMA/CA) method to decide which station in the WLAN should send packets out. The DCF protocol also has an optional virtual carrier sense mechanism that exchanges short request-to-send (RTS) and clear-to-send (CTS) frames between source and destination stations during the intervals between the data frame transmissions.
Under the DCF protocol, the WLAN stations listen to the wireless medium to determine when it is free, or idle. Once a station detects that the medium is idle, the station begins to decrement its back-off counter. Each station maintains a contention window (CW) that is used to determine the number of slot times a station has to wait before transmission. The back-off counter only begins to decrement after the medium has been idle for a DCF inter-frame space (DIFS) period. If the back-off counter expires and the medium is still idle, the station begins to transmit. Thus, it is possible that two stations may begin to transmit at the same time, in which case a collision occurs. If a frame is successfully received by the destination the frame is addressed to, the destination sends an acknowledgement (ACK) frame to notify the source of the successful reception. However, as there is no centralized coordination in DCF, a relatively high number of collisions still occur especially in a WLAN with high load.
In an infrastructure basic service set, typically including an access point and associated non-access point stations, centralized coordination can be achieved when using the PCF protocol as PCF requires the access point to poll the stations to coordinate access to the wireless medium. However, the PCF protocol is rarely implemented as the implementation is complicated because AP has to poll the non-access point stations for the transmission of each polled packet. Additionally, it is difficult to integrate an effective power saving mode with PCF.
The IEEE 802.11e enhances the DCF and PCF protocols through the hybrid coordination function (HCF). The HCF includes two protocols of channel access: enhanced distributed channel access (EDCA) and HCF controlled channel access (HCCA). As the HCCA protocol uses contention free periods to transmit data streams, it makes efficient use of the bandwidth. However, implementation of HCCA tends to require a relatively complex scheduler and added complexity. With the EDCA protocol, data streams are assigned different priorities and thus high-priority data streams have a higher chance of being transmitted than low-priority data streams. Each priority level is assigned a transmit opportunity (TXOP), which is a time interval during which a station in the WLAN can send as many frames as possible.
Audio and video streams require reliable transmission and deterministic delay. At current time it is difficult, however, to provide reliable audio/video transmission in an IEEE 802.11-based WLAN because of insufficient support of broadcast and multicast transmissions. For example, an intra-basic service set collision occurs if associated stations transmit frames when the access point is transmitting broadcast or multicast frames. When two or more basic service sets overlap each other, an overlapping basic service set (OBSS) or inter-basic service set collision may occur. The collision may happen, for example, if a station or the access point of a first basic service set transmits frames when an access point of a second basic service set that overlaps the first basic service set is transmitting broadcast/multicast frames. Furthermore, there is no feedback mechanism to indicate the correct reception of the broadcast/multicast frames transmitted by the access point.
In addition, the EDCA protocol, as it stands, does not provide deterministic delay. Although there are efforts made to include a reservation-based medium access method in the IEEE 802.11n and 802.11s drafts, a number of issues still need to be addressed in order to provide reliable reserved medium access for transmission of audio/video streams. For example, issues related to coexistence with legacy overlapping basic service set and coexistence with or without overlapping basic service set should be considered.